Outputs from a microphone (in particular, a condenser microphone) sometimes include wind noise and/or vibration noise. In order to reduce the noise, a filter circuit is disposed in a preceding stage of an output circuit of the microphone. Since the wind noise and vibration noise are mainly composed of low frequency components, the filter circuit used is a high-pass filter (a low-cut filter).
A condenser microphone has high output impedance. Thus, an impedance converter is provided on the output side of the condenser microphone to reduce the output impedance. The impedance converter mainly includes a field-effect transistor (FET). The high-pass filter that attenuates the low frequency components is disposed in a subsequent stage of the impedance converter and in a preceding stage of the output circuit of the microphone (refer to PTL 1, Japanese Unexamined Patent Application Publication No. 2001-238287).
FIG. 9 is a circuit diagram illustrating an example configuration of a conventional microphone. As shown in FIG. 9, a conventional microphone 100 includes a microphone unit 1 that is a condenser microphone unit, an impedance converter 2, a high-pass filter 30, and an output amplifier 4.
The output from the microphone 100 is a balanced output. The output terminal of the microphone 100 has therefore three pins, i.e., a HOT terminal 5, a COLD terminal 6, and a ground terminal 7. The HOT terminal 5 outputs a positive phase of output signal from the microphone unit 1. The COLD terminal 6 outputs a negative phase of output signal from the microphone unit 1.
In order to match the output impedance of the microphone unit 1 and the input impedance of the high-pass filter 30, it is necessary to reduce the input impedance of the high-pass filter 30 in accordance with the output impedance of the microphone unit 1 that has been lowered by the impedance converter 2. Unfortunately, such a reduction in input impedance of the high-pass filter 30 leads to distortion of output signals from the impedance converter 2.
The high-pass filter 30 also has high output impedance. Thus, a buffer amplifier including an emitter follower circuit with transistors is employed as the output amplifier 4 disposed in a subsequent stage of the high-pass filter 30. In the output amplifier 4, however, a noise level increases due to the high output impedance of the high-pass filter 30. The output impedance is high at frequencies below the cutoff frequency of the high-pass filter 30 and thus the noise level is significantly high at frequencies below the cutoff frequency of the high-pass filter 30.
The high-pass filter 30 includes a capacitor C30 connected in series with an output terminal of the microphone unit 1 and a resistor R30 connected in parallel with the output terminal of the microphone unit 1. If the frequency of the output signal from the microphone unit 1 is low, then the impedance of the capacitor C30 is dominant and the impedance of the high-pass filter 30 is high. Low frequency signals cannot therefore be output toward the output amplifier 4.
If the frequency of the output signal from the microphone unit 1 is high, the impedance of the capacitor C30 is low and high frequency signals can be output toward the output amplifier 4. The high-pass filter 30 outputs signal components with frequencies higher than a certain frequency and cuts off signal components with frequencies lower than the certain frequency. The boundary frequency of the signals output toward the output amplifier 4 in the high-pass filter 30 is called a cutoff frequency.
If the frequency of the output signal from the microphone unit 1 is higher than the cutoff frequency, then the impedance of the capacitor C30 is a negligible level. In this case, the impedance of the resistor R30 is dominant in the high-pass filter 30. When the frequency of the output signal from the microphone unit 1 is higher than the cutoff frequency, the output impedance toward the microphone unit 1 relative to the output amplifier 4 is approximately equal to the output impedance of the resistor R30. As the impedance of the resistor R30 increases, the noise level output from the microphone unit 1 increases. The impedance of the resistor R30 in the high-pass filter 30 is generally higher than the output impedance of the impedance converter 2. Thus, if the high-pass filter 30 is disposed in a preceding stage of the output amplifier 4, then the noise level output from the output amplifier 4 increases as the frequency of the output signal from the microphone unit 1 increases.
The output impedance of the output amplifier 4 corresponds to the output impedance of its preceding circuit multiplied by the reciprocal of the current amplification factor (hFE) of the transistor used when the output amplifier 4 includes an emitter follower. Thus, in an example case that the high-pass filter 30 is not employed, at an output impedance of the microphone unit 1 of 10 Ω and an hFE of the transistor of 100, the output impedance of the output amplifier 4 is 1/10Ω. As described above, if the high-pass filter 30 is employed, at a frequency of the output signal of the microphone unit 1 higher than the cutoff frequency, the resistance component of the high-pass filter is dominant. Hence, the impedance of the microphone unit 1 relative to the output amplifier 4 depends on the value of the resistor R30 in the high-pass filter 30. Assuming that the resistance value of the resistor R30 is 10 kΩ, the output impedance of the output amplifier 4 is 1 kΩ in the above-mentioned case.
If the output impedance of the output amplifier 4 is 1 kΩ, then external noise having a frequency of approximately 50 Hz is electrostatically coupled with a microphone cord (not shown) and is readily output from the output amplifier 4. As a result, noise can be readily mixed into the output of the microphone unit 1.
In order to solve the above-mentioned problems, it is desirable to provide a microphone that does not produce distortions of outputs even if the impedance of a circuit connected to a subsequent stage of the impedance converter 2 is low. It is also desirable to provide a microphone that does not produce any noise due to the impedance of the filter circuit. In addition, it is desirable to provide a microphone that has low output impedance even if the frequency of the output signal is less than the cutoff frequency of the filter circuit.